1. Field of the Invention
This invention relates to a novel process for removing a non-aqueous solvent from a web material such as paper, and optionally thereafter recovering the solvent. In another aspect, this invention relates to a process for removing a non-aqueous solvent from coated or impregnated paper using the latent heat from a condensable vapor. Preferably, the heat transfer vapor used in the solvent removal process is comprised of substantially saturated steam.
2. Description of the Prior Art
The quest for a more efficient and effective process for removing non-aqueous solvents, e.g., organic solvents, from a web material such as paper, cloth and non-woven or woven fabrics, has been continuous. This is particularly true for areas of technology employing commercial processes involving the impregnation of a web substrate with a resin wherein the resin is applied as a solution. The solution generally comprises a non-aqueous, organic solvent, removal and recovery of which is desirable for economic and environmental reasons.
The paper industry is one specific industry where the need for a more efficient and effective process for the removal of non-aqueous solvents is pronounced. More particularly, such need is especially pronounced in that portion of the paper industry pertaining to the impregnation of a paper substrate with phenol-formaldehyde resin. The phenol-formaldehyde resin is generally applied in an alcohol, e.g., methanol solution, with the methanol solvent being removed in a hot air oven. Compare U.S. Pat. No. 2,991,194 issued to Cambron, July 4, 1961.
The use of a hot air oven in the removal of such a solvent, however, has many disadvantages. For example, there is a danger of explosion due to the presence of oxygen. To reduce this danger, the volume of air that is heated and employed is generally verv large in order to provide sufficient dilution of the organic solvent to keep the concentration of the alcohol below the Lower Explosive Limit (LEL). The LEL is the concentration of the organic vapor below which, upon ignition, there is insufficient energy to propagate an explosion. Maintaining an air oven with sufficient dilution to stay below the LEL requires heating large quantities of air which is energy inefficient.
Other disadvantages lie in the cost and difficulty of recovering the solvent, e.g., alcohol solvent. Once the hot air has removed the solvent, the recovery of the alcohol from the hot air requires cooling the air and passing the cooled gas through a bed of activated charcoal on which the alcohol is adsorbed. The charcoal bed is then stripped of solvent by using steam, the steam-solvent vapor mixture is condensed, and finally, the steam-solvent mixture is distilled to recover water and solvent. However, because of the difficulties and expense involved in recovering the alcohol solvent by the foregoing, and other known methods of recovery, the alcohol vapors originating from the resin are generally exhausted to the atmosphere, or incinerated without recovery, in the hot air oven exhaust gas. Exhaustion to the atmosphere produces a severe environmental impact. Incineration is undesirable because it is expensive, capital intensive and can cause odor problems.
Other workers have employed steam as a heat source for evaporating organic solvents from various substrates. Note, for example, U.S. Pat. No. 1,261,005 issued to Barstow et al; U.S. Pat. No. 2,174,170 issued to Schweizer; U.S. Pat. No. 2,590,850 issued to Dungler; and, U.S. Pat. No. 3,089,250 issued to Victor.
U.S. Pat. No. 1,261,005 discloses the use of steam or some other vapor for the removal of volatile solvents from liquid or solid materials. In particular, the process is used in the extraction of oils from particulate solid material. The process disclosed passes the particulate material in a continuous current through a contacting chamber, with the steam or other vapor being passed countercurrently to the solid material. The usefulness of steam in the removal of solvents from a continuous web substrate such as paper is not discussed or considered in the patent.
U.S. Pat. No. 2,174,170 uses steam in a concurrent fashion, i.e., the steam being admitted at the point where the web enters the contacting chamber, vis-a-vis a countercurrent contact where the steam is admitted at the point where the web exits the chamber. Also, the resin system used in the patent is one which is very tolerable of water, e.g., the resin will not precipitate with water, and indeed, contains water.
Another patent, U.S. Pat. No. 2,565,152, issued to Wachter et al, discloses the use of steam in treating fibrous materials with phenolic resins. In the process disclosed, the resin solution is applied to the fibrous material and then steamed. The amount of steam used varies from 1% to 10% of the weight of the resin varnish. Improved impregnation of the resin varnish is the main objective and benefit of the steam procedure of Wachter et al. Drying and removal of the solvent is accomplished subsequently in a conventional hot air oven.
The use of saturated steam in the removal of non-aqueous solvents from many substrates has generally been avoided in the past due to several problems. One, the removal of all the solvent has been found to be very difficult and inefficient through the use of saturated steam. Generally, because of the process parameters used and design of the equipment, e.g., concurrent design. Two, the wetting of a substrate, particularly a web material such as paper, by the use of saturated steam is believed to be very harmful, because it was believed that moisture would precipitate a water sensitive resin, e.g., phenolformaldehyde, in the paper, which was thought to be detrimental to the physical properties of the product. Accordingly, the use of superheated steam was attempted.
U.S. Pat. No. 4,421,794, issued to Kinsley, Jr., discloses the removal of non-aqueous solvents from a substrate such as paper by contacting the substrate with a condensable heat transfer medium which is in a superheated state, e.g., superheated steam. The heat transfer medium is maintained in the superheated state throughout the contacting period so as to effect removal of the solvent without concomitant condensation of the medium onto the substrate. The process thereby avoids the problem of wetting a paper substrate, which the industry believes should be avoided otherwise severe damage to the paper substrate can be realized.
Note also U.S. Pat. No. 4,242,808, issued to Luthi, which discloses contacting a paper web directly with superheated steam to evaporate water from the paper web. The paper web drying system includes a direct contact dryer, a superheater such as a heat exchanger, means for feeding the exhaust steam from the direct contact dryer to the heat exchanger, and means for feeding superheated steam from the heat exchanger back to the direct contact dryer. The removal of a non-aqueous solvent is not discussed or disclosed.
Compare also U.S. Pat. No. 2,298,803, issued to Morris, which discloses the use of superheated steam to dry printed matter. The process involves subjecting the surface of the paper upon which the print has been applied to the action of superheated steam.
The problem of removing and recovering all of a solvent most effectively and efficiently from a web material such as paper, however, has heretofore been unsolved. The use of superheated steam can be most expensive because it requires the use of at least 3 lbs of superheated steam per lb. of solvent removed and it requires the use of a superheater. The use of saturated steam would be more economical in terms of steam use and energy.
Accordingly, it is a major object of this invention to provide a novel process devoid of the aforementioned disadvantages wherein steam is employed.
Another object of this invention is to provide a method for thermally treating a web material, such as paper, from which volatile substances of inflammable character are to be removed in such a manner that all danger of explosion is averted.
Yet another object of this invention is to provide a quick and efficient process for the removal and recovery of a solvent from a web material without exhausting organic substances to the atmosphere.
Another object of this invention is to treat an impregnated or coated web from which a non-aqueous volatile solvent is to be removed with a heat transfer vapor which also performs as a solvent vapor carrier.
Still another object of this invention is to provide an easy and efficient method for recovering the organic solvent from a water insoluble resin coated paper.
Other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a study of the disclosure, the appended claims and the drawing.